Similar to most other morphogen and growth factor signaling pathways, mammalian Wnt signaling is deployed multiple times during development and tissue homeostasis through the use of 19 different ligands, 10 receptors, and multiple coreceptors, including LRP5/6, Ror1/2, and Ryk (van Amerongen and Nusse, 2009). In addition, different secreted antagonists that bind either Wnts, such as SFRP1/2/3/4/5 and WIF1, or LRP5/6, including DKK1/2/4 and SOST, modulate interactions between ligands and receptors. These membrane and extracellular proteins and their multiple isoforms provide for differential regulation at the level of expression and combinatorial protein interactions. Most Wnt isoforms appear to be capable of binding coreceptor LRP5/6, and LRP5/6 engagement specifies canonical, or β-catenin dependent, Wnt signaling. Wnt heterodimerizes LRP5/6 and FZD to mediate phosphorylation of the LRP5/6 intracellular domain and Axin binding (Tamai et al., 2000; Semenov et al., 2001; Tamai et al., 2004). DVL is brought into the complex by directly binding both Axin and FZD, and DVL oligomerization likely enlarges these protein complexes on the cytoplasmic face of the membrane that sequester GSK3 and inhibit its phosphorylation and destabilization of β-catenin (Mi et al., 2006; Bilic et al., 2007; Schwarz-Romond et al., 2007; Cselenyi et al., 2008; Piao et al., 2008; Zeng et al., 2008; Wu et al., 2009).
The uniquely large number of ligand isoforms, displaying considerable primary sequence divergence, that mediate mammalian canonical Wnt signaling contrasts with the pair of highly homologous coreceptors. The LRP6 and LRP5 extracellular domains consist largely of four homologous regions, named E1 to E4 from N- to C-terminal, each containing a YWTD-type β-propeller and EGF-like domain (Jeon et al., 2001). Each repeat at a similar position in LRP6 and LRP5 is highly conserved, whereas the different repeats within the same protein are considerably more divergent. Interestingly, Bourhis et al. (2010) demonstrated that Wnt9b binds exclusively within the E1-E2 region in vitro, whereas Wnt3a binds only to a fragment containing E3-E4, suggesting that each repeat, or a combination of two adjacent repeats, binds to a different subset of Wnt isoforms. This arrangement may accommodate the diversity of Wnt proteins, and possibly also allow for their differential regulation by LRP5/6 antagonist ligands. In Notch and VEGF receptors, whose extracellular regions contain repeats of EGF-like and Ig domains, respectively, binding of multiple ligand isoforms is localized to the same region of one or two repeats, although the presence of other repeats can enhance binding. (Rebay et al., 1991; Davis-Smyth et al., 1996; Cunningham et al., 1997).
For receptor tyrosine kinases, ligand-induced dimerization initiates stimulation of the kinase activity and signaling. While ligand-induced receptor-coreceptor heterodimerization is necessary for canonical Wnt signaling, there is no clearly defined role for LRP5/6 or FZD homodimerization. Forced dimerization of different recombinant LRP6 proteins can either activate or inhibit Wnt signaling.
β-catenin-dependent Wnt signaling is initiated by a Wnt isoform binding to both the receptor FZD and coreceptor LRP5/6, which then assembles a multimeric complex at the cytoplasmic membrane face to recruit and inactivate the kinase GSK3. Whether and how mechanistically different interactions between Wnt isoforms and receptors might modulate this process remains to be determined.